The evaporation phenomenon of a liquid droplet was investigated by using microfabricated heaters. All 32 microheaters were designed to have the same resistance. Gold microheaters worked both as temperature indicators and as heaters. The first experiment was performed under a constant voltage mode to investigate the temperature and heat flux variation of the heated surface by the evaporating droplet. The second experiment was performed under constant temperature mode to investigate the spatial and temporal heat flux variation of the constant temperature heater surface by the evaporating droplet heater. Droplet evaporation was recorded with a CCD camera. Experimental data showed temperature and heat flux variations inside and outside of the droplet with respect to time and radial position from the center of the droplet by tomographic deconvolution.

The evaporation phenomenon of a liquid droplet was investigated by using microfabricated heaters. All 32 microheaters were designed to have the same resistance. Gold microheaters worked both as temperature indicators and as heaters. The first experiment was performed under a constant voltage mode to investigate the temperature and heat flux variation of the heated surface by the evaporating droplet. The second experiment was performed under constant temperature mode to investigate the spatial and temporal heat flux variation of the constant temperature heater surface by the evaporating droplet heater. Droplet evaporation was recorded with a CCD camera. Experimental data showed temperature and heat flux variations inside and outside of the droplet with respect to time and radial position from the center of the droplet by tomographic deconvolution.

Fast mixing remains a major challenge in droplet-based microfluidics. The low Reynolds number operating regime typical of most microfluidic devices signify laminar and orderly flows that are devoid of any inertial characteristics. To increase mixing rates in droplet-based devices, a novel technique is presented that uses a high Reynolds number gaseous phase for droplet generation and transport and promotes mixing through binary droplet collisions at velocities near 1m/s. Control of multiple gas and liquid streams is provided by a newly constructed microfluidic test bed that affords the stringent flow stability required for generating liquid droplets in gaseous flows. The result is droplet production with size dispersion and generation frequencies not previously achievable. Limitations of existing mixing diagnostic methods have led to the development of a new measurement technique for measuring droplet collision mixing in confined microchannels. The technique employs single fluorophore laser-induced fluorescence, custom image processing, and meaningful statistical analysis for monitoring and quantifying mixing in high-speed droplet collisions. Mixing information is revealed through three governing statistics that that separate the roles of convective rearrangement and molecular diffusion during the mixing process. The end result is a viewing window into the rich dynamics of droplet collisions with spatial and temporal resolutions of 1μm and 25μs, respectively. Experimental results obtained across a decade vi of Reynolds and Peclet numbers reveal a direct link between droplet mixing time and the collision convective timescale. Increasing the collision velocity or reducing the collision length scale is the most direct method for increasing droplet mixing rates. These characteristics are complemented by detaching droplets under inertial conditions, where increasing the Reynolds number of the continuous gaseous phase generates and transports smaller droplets at faster rates. This work provides valuable insight into the emerging field of two-phase gas-liquid microfluidics and opens the door to fundamental research possibilities not offered by traditional oil-based architectures. / text

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This project evaluated the metal transfer behaviour of the variable polarity (VP) GMAW process. Analysis was performed using high speed video that was synchronised with high speed data acquisition. Melting rate measurements were found to be very dependent on current waveform, polarity, and droplet size, and metal transfer if it occurred, for each waveform period. The transient conditions of current waveform and metal transfer produced rapid changes in arc behaviour which influenced the melting at the electrode tip and growing droplet. The concentrated melting theory was developed to explain the significant increase in electrode extension burnoff and droplet growth rate that occurred at short EN time as a function of current, and during EP peak pulse when the pre-pulse droplet volume was small. The highest electrode extension burnoff and droplet growth rate occurred when the arc was permitted to climb over the solid electrode tip producing rapid concentrated melting. Likewise, large molten droplets were found to promote a negative electrode extension burnoff and a decreased droplet growth rate. The arc rooted on large droplets providing additional heating but limited electrode melting. The droplet burnoff rate (DBR) method was developed and found to yield good experimental measurements for the arc and resistive heating coefficients used in a 2nd order melting rate equation developed for a complex waveform process, like VP-GMAW. For the EN period, the EN time affected the melting rate as a function of EN current. The greater melting rate that occurred at low EN time was measured by the changes in the resistive heating coefficient. Concentrated arc melting of the electrode extension at low EN time caused the slope of the burnoff diagram to increase, which represented the resistive heating coefficient. The melting rate of the EP pulse was related to the pre-pulse droplet volume. Large pre-pulse droplets decreased the arc heating coefficient, which could be negative, which meant the electrode extension was increasing and the arc length was decreasing in that waveform period. VP-GMAW power supplies offered stable operation for welding sheet structures on both carbon steel and stainless steel. Higher travel speeds were required as the %EN of the waveform increased to produce acceptable constant deposit area fusion. Welding speeds were up to 300% higher with VP-GMAW compared to the GMAW-P process when welding lap joints on 1.8 mm thick material with a 1.8 mm gap. VP-GMAW heat input was up to 47% less than GMAW-P for the same melting rate.

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A test rig was developed in an effort to perform droplet evaporation and combustion experiments at high levels of turbulent intensity under elevated pressures and temperatures. The detailed explanation of the design and operation of the various components that are part of the testing apparatus is presented. Once the apparatus was completed, 2D Laser Doppler Velocimetry measurements were used to fully characterize the turbulent field inside the chamber. The results showed that the test rig was capable of producing homogenous isotropic turbulence with a 40 mm central region of the chamber at turbulent kinetic energy levels of up to 5.0 m/s. From the characterization data a correlation of turbulent kinetic energy vs fan speed was produced. The produced correlation is valid at standard conditions as well as elevated pressures and temperatures. After determination of the turbulent field, droplet evaporation experiments were performed, first at standard conditions and then elevated temperature and pressure. The results show that turbulence continued to enhance droplet evaporation at elevated temperature and pressures, 298-348°K and 1-21 bar respectively. Broad conclusions are then drawn from the work performed in the study and recommendations are made for future work and improvements to the test apparatus.

A test rig was developed in an effort to perform droplet evaporation and combustion experiments at high levels of turbulent intensity under elevated pressures and temperatures. The detailed explanation of the design and operation of the various components that are part of the testing apparatus is presented. Once the apparatus was completed, 2D Laser Doppler Velocimetry measurements were used to fully characterize the turbulent field inside the chamber. The results showed that the test rig was capable of producing homogenous isotropic turbulence with a 40 mm central region of the chamber at turbulent kinetic energy levels of up to 5.0 m/s. From the characterization data a correlation of turbulent kinetic energy vs fan speed was produced. The produced correlation is valid at standard conditions as well as elevated pressures and temperatures. After determination of the turbulent field, droplet evaporation experiments were performed, first at standard conditions and then elevated temperature and pressure. The results show that turbulence continued to enhance droplet evaporation at elevated temperature and pressures, 298-348°K and 1-21 bar respectively. Broad conclusions are then drawn from the work performed in the study and recommendations are made for future work and improvements to the test apparatus.

No description available.

The integration of droplet microfluidics devices with nanopore sensors offers a powerful and miniaturized sensing platform. Such devices can utilize the pre-processing capabilities of microfluidics in conjunction with single molecule sensing offered by nanopores. Microfluidics devices utilizing segmented flow (droplets) allow the compartmentalization of chemical and biological reagents in droplets reducing the processing time and associated cost, which is advantageous to biomolecular applications. Droplet microfluidics have been used in diagnostics and therapeutic applications such as cell and biomarker detection, gene amplification, and drug delivery. Nanopore sensors are currently used in investigating DNA and gene detection, protein-protein interactions, protein folding, and enzymatic kinetic reactions. This thesis proposes a design and outlines a methodology to integrate nanopore sensors within a droplet microfluidic device. The chapters are organized in highlighting three main objectives. The first objective is creating the segmented flow of oil-KCl droplets using a T-junction microfluidic design. The second objective is measuring the conductance of the segmented flow prior to the nanopore integration by using two side channel-AgCl electrodes. Subsequently, the third objective is integrating the droplet microfluidic device with a silicon nitride chip for nanopore fabrication. The nanopore is then created using controlled dielectric breakdown (CBD) method for DNA detection within droplets. The results show the feasibility of sensing individual DNA molecules within droplets using a nanopore sensor. The implemented approach expands upon nanopore applications to detect different samples simultaneously, fast food-borne pathogens and tumor discrimination in cancer biology. We anticipate that this integration is the future of nanopore sensors.